Which font-display value to use?
This depends a lot on how you are using your webfont, and whether rendering the text in a fallback font makes sense. For example, if you're rendering the main body text on a site, you should use
font-display: optional. On browsers that implement it, like Chrome, the experience will be much nicer: your users will get fast content, and if the web font download takes too long, they won't get a page relayout halfway through reading your article.
If you're using a web font for icons, there is no acceptable fallback font you can render these icons in (unless you're using emoji or something), so your only option is to completely block until the font is ready, with
font-display: block.
See also